The present invention relates to optical disks, that is to say disks on which information data is recorded, this data being readable by means of a focused ray beam. This data may represent video information, i.e. image, sound and data signals or purely digital information in data processing or analog signal digitalization applications. The invention relates more particularly to a process for manufacturing such disks.
Optically readable disks are generally such that the information is recorded on a track in the form of micro-reliefs whose width is of the order of a micrometer and whose length is fixed or variable depending on the recording process. This track may have the form of concentric circles or may be a spiral track, the pitch between the grooves in the radial direction being between 1.5 and 2 micrometers. Such disks allow a great amount of optically readable information to be recorded. Reading is obtained by diffraction of radiation concentrated on the plane containing the information when the disk is rotating, the reading device comprising radial and vertical movement control devices for maintaining the focusing tip of the reading beam on the track. Reading may be carried out by transmission or reflection.
The disk may comprise one or two recorded faces.
Though an original recording may be obtained directly by a process using recording means also comprising a focused ray source, the same can not be said for duplicating this recording.
In fact, if a number of copies of this recording is desired, it is then necessary to use processes based on duplication using one (monoface disk) or two masters (double face disk) reproducing the original recording.
Numerous methods have been proposed in the prior art.
Two types of disks may be generally distinguished, at least in so far as the thickness is concerned; these are called "thick" disks, that is to say having typically a thickness greater than about 300 micrometers, as opposed to so called "thin" disks. Although this distinction is essentially conventional, it corresponds, considering the techniques and materials used, to technological difficulties of different magnitudes met with during the manufacturing procedure.
In so far as the "thick" disks are concerned, there exist two main thermal processes:
a compression process which consists in placing a preheated blank of thermoformable material between two masters one at least of which carries an impression to be duplicated, and heating and pressing until the material has flowed and has taken on the whole of the impression; the thickness of the disk is determined by a compromise between the pressure exerted and the fluidity of the thermoformable material;
an injection process which consists in forcing under a very high pressure an amount of paste of a thermoformable material for filling the space existing between two masters; the thickness of the disk obtained is determined by the compromise between the pressure exerted and the fluidity of the thermoformable material.
In so far as the "thin" disks are concerned, a third method may be added to the two previously mentioned methods which is generally called stamping which consists in placing a sheet of thermoformable material between the two masters carrying the information. The surface is softened by a temperature-pressure cycle so that the impression may be taken up. The final thickness of the disk is determined by the thickness of the sheet.
Finally, for all types of disks, in addition to the above mentioned thermal methods, a different method appeared a few years ago which consists in photopolymerizing, in the presence of radiation in the ultra violet range, a photosensitive material deposited on the substrate by coating and on which the master copies the information while the photopolymer material is still liquid. The thickness obtained is in general slightly greater than the thickness of the substrate.
Experience has shown that the most difficult problem to resolve for obtaining a disk of good quality is certainly that of the inherent flatness of the disk.
The type of inherent flatness naturally influences the final quality of a disk. It is relatively easy to correct a defect which is in the form of a single undulation per revolution or buckling, even of large amplitude, by means of the servomechanisms with which the reading means are provided. In fact, in this case, the defect is associated with a very low frequency for which the gain of the servomechanisms is at its maximum.
On the other hand, when the number of defects, even of relatively small amplitudes, is high and they are distributed in a random fashion, the gain of the servomechanisms decreases because of their limited pass bands and these defects cannot be corrected. It is then necessary to obtain a surface quality condition sufficient to overcome this phenomenon.
The compression or injection processes which require high power presses often introduce very high stresses into the materials and at the present time the best compromise between all the specifications and in particular those related to optical parameters, leads to using methyl polymethachrylate ("PMMA").
The photopolymerization method which is certainly less restrictive, both for the material of the disk and for the masters, requires even so the use of a good quality substrate. Now, this latter is often obtained by injection or compression and so the same difficulties arise at the time of its manufacture.
As opposed to that, the stamping method is advantageous first of all because pre-pressing may be carried out with masters without information so as to improve the inherent flatness but also because there is practically no movement of material under the master. This method gives good results for thin films typically less than 300 micrometers as was mentioned above. When it is desired to use greater thicknesses, these are "plates" and no longer films. The "plates" generally have a fairly mediocre inherent flatness and this method is inapplicable to so called "thick" disks.
The invention relates to the manufacture of disks of this latter type and aims at overcoming the difficulties which have just been set forth.
To this end, the object of the invention is then an optical disk obtained by this process.